The invention relates to a process for the photo-optical measurement of the absorption behavior of solid, liquid and gaseous media, especially in the photometric analysis of gases by means of infrared rays, by exposing the analysis medium to a measuring light beam made intermittent by a rotating chopping device, the chopper frequency being maintained constant by regulating the speed of the chopping device.
The use of intermittent measuring light beams is common in general in the photo-optical measurement art. Also, the expression, "light modulation," is used, and the devices used for this purpose are called choppers. Light modulation is performed mainly for the purpose of separating the actual measuring signal from signals known as "noise signals," the noise signal being given a much lower frequency than the measuring signal. While the noise signal fluctuates only over relatively long time periods and therefore can be looked upon as a virtually direct-current signal, the chopper frequency is selected on the basis of the measuring device that is being used. For semiconductor detectors it is in the kiloherz range, and for gas detectors using, for example, membrane condensers or anemometers, it is in the range of about 5 Hz with an increasing tendency towards about 500 Hz and over.
The subject of the invention is preferentially applied to the analysis of gas by determining the absorption of an infrared light beam. Such processes and apparatus are the state of the art (German "Auslegeschrift" Nos. 1,296,839 and 1,698,218).
Light choppers or light modulators for photometers are known in numerous versions. In most cases they involve an electric motor on whose shaft there is fastened a chopper wheel provided with openings. The measuring effect at the detector of the photometer is dependent in numerous applications on the chopper frequency, so that usually a synchronous motor is used to drive the chopper. Chopper wheel drives are also known in which the motor is a commutator-less direct-current motor whose rotatory speed is held constant by a regulating system. The rotatory speed information that is used for the regulation is obtained from the voltage of the motor winding. Furthermore, light choppers have been known in which the modulation of the light is produced by a rapidly swinging pendulum or by forks-- called tuning forks-- which are excited at their natural frequency. Deficiencies of such apparatus are to be found in their physical size and in the fact that, when the chopper wheel is driven by a motor, the beam path is almost always partially in communication with the interior of the motor, which can result in disturbances of the measuring signal due to outgassing effects, especially when the chopper is used in gas analysis apparatus. In a number of known apparatus, the phase signal is derived from the position of the chopper wheel by means of what is known as a rotation indicator, such as for example a photoelectric cell. The phase signal can furthermore also be derived from the drive voltage or, in the case of commutator-less direct-current motors, from the motor winding voltage. Phase rotations, which occur in the detector and in the amplifier that follows the detector, are not taken into consideration in this method and therefore can have an adverse effect on the accuracy of measurement.
Consideration must be given to the fact that the measuring signals which are obtained at the detector in high-resolution photometers, for example, are very small in proportion to the noise signals which always occur, so that phase-controlled rectification must usually be used for the separation of the signals. Generally a high resolution is expected from measuring apparatus of the kind under discussion, i.e., a high response sensitivity to extremely small changes in the specimen concentrations, down to 10.sup.2 ppm.
An improvement of the evaluation of the measuring signal in non-frequency-dependent detectors can be accomplished by an increase in the chopper frequency. The well-known sensitivity of gas detectors and transducing circuitry to mechanical vibration is decidedly reduced in this manner. At high chopper frequencies, very large diameters are very soon required, since the speed of the motors must be kept relatively low on account of bearing problems and the trouble involved in balancing. Large chopper wheels, however, are undesirable because they militate against compactness.